The present invention relates to a process for separating low molecular weight alcohols from aqueous solutions. It is possible to obtain low molecular weight alcohols having a concentration above about 95% and in many cases above about 99.9% by utilizing the process of this invention. The process is especially suitable for separating ethanol from aqueous solutions.
The concentration or complete recovery of ethanol from aqueous solutions has been accomplished by distillation for many years in the production of alcoholic beverages, solvents and a variety of chemicals. Ethanol from such distillations, either alone or in combination with hydrocarbon fuels such as gasoline, has been more recently used as an automotive fuel. Contamination by residual water is an undesired consequence of most simple fractional distillation schemes. Contamination by residual water is especially undesirable when the alcohol is to be used as fuel.
Extraction of aqueous ethanol solutions by organic solvents has been previously proposed. In these proposals, the extract containing both ethanol and solvent is generally distilled to separate the product ethanol and a recycle solvent. An article by J. W. Roddy, entitled "Distribution of Ethanol-Water Mixtures to Organic Liquids" in Ind. Eng. Chem. Process Des. Dev., volume 20, pp 104-108 (1981) indicates that a wide variety of organic solvents have been used for such extractions, but that the number of solvents having distribution coefficients greater than 0.5 for ethanol and separation factors greater than 10 from aqueous solutions (as defined in the Roddy article) are quite limited. The article indicates the general order of extraction for ethanol to be hydrocarbon=halocarbon&lt;ether&lt;ketone&lt;amine&lt;ester&lt;alcohol=phosphate. The best candidate identified in the article was 2-ethyl-1-butanol, which had a distribution coefficient of 0.69 for ethanol and a separation factor of 30. The next best candidate, tri-isobutyl phosphate, had a distribution factor for ethanol of 0.65 and a separation factor of 10. The solvents that are proposed in the present invention have distribution coefficients of 1.1 or even larger for ethanol.
It has also been proposed to conduct a distillation to separate ethanol from water with an additional solvent being added to the system so as to either enhance the separation and purity of ethanol as the overhead and of water as the bottoms, or to reverse the volatilities for ethanol and water, causing water to be removed as top product, and ethanol mixed with solvent to be removed as bottom product. Examples of such suggestions are contained in U.S. Pat. No. 2,591,672 (with a hydrocarbon as the extractive distillation solvent) and an article by C. Black entitled "Distillalation Modeling of Ethanol Recovery and Dehydration Process for Ethanol and Gasohol", in Chem. Eng. Prog., September 1980, pp 78-85, especially at pp 82-84. It has also been proposed in an article by S.A. Leeper and P. Wankat, in Industrial and Engineering Chemistry Process Design and Development, April 1982, pp 331-334 to extract alcohol from an aqueous solution with gasoline. However, this process requires concentrating the alcohol in the feed stream to a 90% level prior to the extraction. Also, said process produces only gasohol, a mixture of gasoline and alcohol, and does not produce pure alcohol.
Copending Application Ser. No. 276,302 (Zudkevitch et al., 1981), and now-abandoned discloses a process for the separation of ethanol from aqueous solutions by reversing the relative volatility between ethanol and water. In the Zudkevitch et al. procedure and in the procedures discussed by Black, the ethanol/water, ethanol/solvent and solvent/water binaries all exhibit positive deviations from ideal mixing. Positive deviations from ideal mixing for the solvent/alcohol binary are less desirable than negative deviations when extraction of the alcohol is considered, because said positive deviations are accompanied by repulsive forces between the alcohol and the solvent. Thus, separation of the alochol from water by extraction or extractive distillation is more difficult when the alcohol/solvent binary exhibits a positive deviation from ideal mixing. On the other hand, in the process proposed herein, separation of alcohols from aqueous solutions is facilitated when the water/solvent binary exhibits a positive deviation from ideal mixing.
While the above references indicate the desirability of extraction, extractive distillation and azeotropic distillation schemes to recover ethanol from aqueous solutions, a need exists for solvents suitable for carrying out the desired separation with higher selectivities than those proposed heretofore. We have discovered a class of phenolic solvents which have negative deviations from ideal mixing for the ethanol/solvent binary. Negative deviations from ideal mixing for the ethanol/solvent binary are accompanied by attractive forces instead of repulsive forces. Thus, the phenolic agents are highly suitable for the extraction or extractive distillation of solutions comprised of alcohol and water, especially solutions of ethanol and water.